A high-end VR headset is an investment in your gaming space. Buying the right model once means you won't need an upgrade for years, so don't settle for entry-level gear that will frustrate you in six months.
Tech cycles move fast, but the best deals usually land during holiday sales. Wait for those dips, and if you can, grab last year's flagship model when it goes on clearance; you will get better performance for your money than a brand new mid-tier unit.
Remember that the 15-month window is a deadline, not a pass to ignore your balance. If you still have a remaining amount on the card after the 15 months are up, you will start paying interest on whatever is left over.